The present invention relates to variable cam apparatus primarily for implementing variable valve timing techniques in internal combustion engines, and more particularly to such apparatus employing rollers.
Mechanisms using a camshaft capable of varying the timing of valves, i.e., when the valves open and when the valves close, in relation to the movement of the crankshaft, and how long they remain open or closed, are known. In some devices, variable timing is obtained by controlling the cam follower geometry. U.S. Pat. No. 4,357,917 to Aoyama discloses a number of different geometries for the nose portion of a cam (FIGS. 9A, 9B, and 9C) used in a variable valve timing system for induction control of an internal combustion engine.
In a number of the variable valve timing mechanisms, a cam lobe is movable relative to the outer cylindrical surface of the camshaft. Such designs include U.S. Pat. No. 4,770,060 to Elrod et al, U.S. Pat. No. 4,771,742 to Nelson et al, and U.S. Pat. No. 4,917,058 to Nelson et al, which patents are hereby incorporated herein by reference. In many of these movable lobe camshafts, the cam follower is a flat surface disposed at the end of a valve tappet. Examples of this design are shown in U.S. Pat. No. 2,888,837 to Hellmann, DE 3,234,640A to Kruger, DE 2,921,645 to Lehr, French Patent No. 1,109,790 to Rooy, Patentschrift 704,575 to Von Ruti, and U.S. Pat. No. 1,527,456 to Woydt et al. An alternative design uses followers with a curved surface. Examples of curved follower surfaces are shown in DE 3212-663-A to Martin (FIG. 1), U.S. Pat. No. 4,388,897 to Rosa (FIG. 7), and U.S. Pat. No. 4,770,060 to Elrod et al (FIG. 3). In some such curved follower designs, DE 3212 -663-A to Martin for example, the base of the cam rides above the outer cylindrical surface of the camshaft and meets the circular surface tangentially and continuously so that a follower encounters little or no discontinuity upon leaving the cam surface and engaging the camshaft surface where the cam surface ends.
As known in the art, the reduction of friction in the valve train can result in more efficient engine output. The use of roller followers can reduce friction where the cam surface meets the follower surface. In a further alternative design, roller followers are used to engage the surface of a movable cam. Examples of the roller follower designs include a multi-linkage design shown in the top center FIG. 10 of an article entitled "Variable Valve Timing for IC Engines" appearing in Volume 10, No. 4, of Automotive Engineer (August-September 1983), and the more conventional designs shown in Patentschrift 727,987 to Borner, U.S. Pat. No. 862,448 to Cornilleau, and U.S. Pat. No. 733,220 to Krebs. In contrast to the designs employing flat or curved surfaces as cam followers, the devices employing roller followers encounter distinct problems relating to rising and falling movements of the roller followers over short durations. These roller follower movements translate through a suitable linkage to corresponding sudden, brief opening and closing movements of the valves. These valve movements interrupt the rate at which the valve is opening or closing, as the case may be. Such movements either briefly accelerate the trend (opening or closing) of valve movement or decelerate the trend of valve movement. One of these problems occurs when the roller moves between the camshaft's base circle, which is defined by the exterior surface of the camshaft, and either the beginning of the leading edge of the cam surface or the end of the trailing edge of the cam surface.
The problem relating to the base of the cam is illustrated well in U.S. Pat. No. 862,448 to Cornilleau, which uses a roller follower to engage a cam lobe which moves in a radial direction into and out of a slot in the camshaft. As shown in particular in Cornilleau FIGS. 13, 15 and 16, the roller encounters a gap between the riding surface of the cam and the riding surface on the exterior surface of the camshaft.
The problem is also apparent in Patentshrift No. 727,987 to Borner. A U-shaped main cam is fixed to rotate with the Borner shaft, and a secondary cam is connected to a solid shaft inside the hollow main shaft and projects through an opening in the main shaft corresponding to an opening in the U-shaped portion of the main cam. The secondary cam is movable into and out of this opening in the U-shaped portion of the cam. In this way, one can change the length of the cam surface which opens the valve and maintains the valve in the open position. This permits changing the duration of time during which the valve is held open for example. The roller is initially lifted on the full width of the main cam so that during the lifting of the valve, the entire inertia of the control mechanism as well as the tension of the valve springs and the gas pressure against the valve head can be spread over the full width, which thus is available to absorb the forces without an unduly high surface pressure. The linkage roller rolls first of all over the upper head of the main cam. Then the linkage roller moves over the precisely equally high head of the secondary cam. Finally, the linkage roller drops suddenly as it moves over the closing ramp and onto the lower cam surface whereby the valve closes. The failure of the surfaces to match, produces discontinuous roller motion when the linkage roller drops upon the closing ramp onto the lower cam surface. Thus, the otherwise smooth rising and falling motion of the rollers, which translates through a suitable linkage to activate valves for example, becomes subject to sudden discontinuous movements as the roller rides on the transition portions of the cam surface.
Instead of a U-shaped design, Borner's main cam can be designed in a split fashion whereby both halves of both sides can be pushed over the secondary cam. However, this split design does not alleviate the problem just described.
Devices shown in documents DE 3234640A1 to Kruger, DE 2921645A to Lehr, and DE 3212663A1 to Martin, which involve movable cams engaging flat surface followers or curved surface followers, are affected far less than roller followers when engaged by movable cams.
The Martin movable cam 3 has extended ears which overlap and ride on or above an inner sleeve that is integral with the camshaft and the nonmovable cam. The movable cam 3 is keyed to rotate with inner shaft element 2. The valve tappet 4 has a curved follower surface in the shape of a partial circle whose radius corresponds to the elevation of the cam from the axis of the shaft. The follower surface engages the movable cam at a single point, similar to the roller follower. However, when the transition from the movable cam to the base circle of the camshaft must be bridged, the curved surface of the follower engages the movable cam at one point and the base circle of the camshaft at another point. By contrast, the roller follower is incapable of engaging the movable cam at one point and the base circle at another point without riding up and down the valley formed between the base circle and the movable cam. However, as noted above, it becomes desirable if roller followers can be used. Moreover, in such environments, variable valve timing mechanisms employing a movable cam lobe also are desirable.